1. Field of the Invention
The present invention relates to a method for cutting a single crystal ingot wherein a straightly running portion of the endless circuiting blade is used for cutting a single crystal ingot.
2. Description of the Related Art
Conventionally, with regard to cylindrical single crystal ingots which serve as a material of silicon wafers, the CZ method has been mainly used for the manufacture of ingots having a diameter of 8 inch or less. When a cylindrical single crystal ingot is produced by the CZ method, it has its top and tail portions removed before it is sliced into wafers. The remaining body portion is divided by cutting into several blocks and some wafers are sampled from those blocks. In order to remove the top and tail portions of a cylindrical single crystal ingot, and to divide the resulting body portion into several blocks, a cutting machine called a band saw has been used. The band saw type cutting machine comprises a pair of pulleys rotating round their respective central, vertical axes which are separated from each other by a specified distance with an endless belt-like blade is tightly wound around the rims of those pulleys. The blade is obtained by coating diamond particles by electric deposition onto both lower ends of an endless belt. When one pulley is put into rotation, the blade runs a circuit defined by the rims of the two pulleys at a high speed.
When the blade runs along the rim of a pulley, it takes a round course. When the blade runs a space between the two pulleys, it runs a straight course with sideward swings unless otherwise treated. To prevent these sideward swings, blade guides are provided at both ends of the straight course where cutting occurs. Each blade guide has two limbs with a slit in between and the two limbs have carbon shoes in their inner surfaces such that, when the blade guide receives the blade in its slit, the carbon shoes are brought into contact with both surfaces of the blade. This arrangement allows the blade to run a straight course in the same cutting direction constantly. A single crystal ingot is arranged below the straight course of the blade such that the longitudinal axis of the ingot crosses at right angles with the direction of the straight course the blade takes during running. For cutting the ingot, the pair of pulleys are allowed to descend vertically while the blade is running along their rims so that the straightly running portion of the blade is brought into contact with the single crystal ingot below to cut the latter.
Currently, cylindrical single crystal ingot used as a material in the manufacture of silicon wafers comes to have an increasingly large diameter to meet requirements for the low cost production of wafers, because low cost production of wavers leads to the production of competitive semiconductors. Some single crystal ingots currently made have a diameter up to 12 inches. When a band-saw type cutting machine is used to cut cylindrical single crystal ingots having a diameter as large as 12 inch, the cutting machine must have a sufficiently wide blade on an endless belt and two pulleys must be separated sufficiently widely apart to allow the blade to run a sufficiently long straight course to encompass that potbellied ingot. However, if the straight course the blade takes during running is made too long, the blade will be more prone to sideward swings which, if present, will degrade the cut precision of the cutting machine.
In order to meet this problem, a method for cutting single crystal ingot has been proposed (for example, see Patent Document 1). According to the method, to each blade guide are provided a sensor for detecting sideward swings of the blade running nearby, and two fluid jet nozzles facing both sides of the blade, such that if the blade is subject to a sideward swing, and the sensor detects the swing exceeds a certain limit, one fluid jet nozzle at a side to which the blade is swung is activated to jet liquid onto the swung blade to push it back, thereby canceling the swing and recovering the straightness of the course of the blade. It has been reported that according to the method it is possible to cut single crystal ingot comparatively precisely because the sideward swings of the blade can be checked and corrected constantly.
Patent Document 1: Japanese Patent No. 2727421 (Claims)
The blade consists of diamond particle files formed by electric deposition on the lower margins of an endless belt, and thus with the repeated use in the cutting of ingots, the blade is increasingly cleaned of diamond particles as a natural process. However, natural cleaning of diamond particles does not occur evenly over the blade. It often occurs that some parts of the blade are cleaned of diamond particles heavily while other parts remain comparatively unaffected, which may determine a preferred direction for the blade to take during cutting. The blade having a skewed predilection in the cutting direction will degrade the precision of cutting unless properly treated. However, if such a blade is inclined to a preferred direction divergent from a normal specified direction during cutting, it is hardly possible, according to the aforementioned conventional method, to forcibly push back the inclined blade by applying fluid jet to the blade because the inclining tendency of the blade is inherent to the blade itself. This makes it difficult to achieve the stable cutting of ingots. In addition, the pressure wrought by the fluid jet applied to the bent side of the blade is often subject to variation, and if the blade is exposed to such varied pressure, the slice cut by the blade will have cut surfaces with stepwise undulations: the surface of the slice will have a degraded flatness. Moreover, because the pressure wrought by the fluid jet acts in a direction normal to the direction in which the blade runs during cutting, the tension of the blade is enhanced, giving extra stresses to the blade which will shorten the due life of the blade.